Assessing the Interaction Between Geologically Sourced Hydrocarbons and Thermal–Mineral Groundwater: An Overview of Methodologies

Abstract

Groundwater sustains ecosystems, agriculture, and human consumption; therefore, its interaction with hydrocarbons is an important area of research under the umbrella of environmental science and resource exploration. Naturally occurring or anthropogenically introduced hydrocarbons can significantly impact groundwater through complex geochemical processes such as dissolution, adsorption, biodegradation, and redox reactions and can also affect groundwater chemistry in terms of pH, redox potential, dissolved organic carbon, and trace element concentrations. Accurate determination and identification of hydrocarbon contaminants requires advanced analytical methods like gas chromatography, GC–MS, and fluorescence spectroscopy, complemented with isotopic analysis and microbial tracers, which provide insights into sources of contamination and biodegradation pathways. The presence of hydrocarbons in groundwater is a matter of environmental concern but can also valuable data for petroleum exploration, tracing subsurface reservoirs and seepage pathways. This paper refers to the basic need for geochemical investigations combined with advanced detection techniques for successful regulation of thermal–mineral groundwater quality. This contributes towards successful sustainable hydrocarbon resource exploration and water resource conservation, with emphasis on the relationship between groundwater quality and hydrocarbon exploration. The study points out the significance of continuous observation of thermal mineral waters to identify their connection with the specific hydrocarbons of each study area.

1. Introduction

On a global scale, groundwater resources are a strategic water source. Stored within aquifers and abstracted through boreholes, groundwater is used mainly for agricultural purposes, industrial applications, and domestic consumption. Reliance on groundwater is increasing as its quality is generally higher than that of surface waters. Dependence on groundwater is greater in regions that experience frequent drought events or where surface water is contaminated. In the era of climate crisis, groundwater contributes to water safety, especially in severe drought events [1]. Protecting groundwater is critical to ensure its availability and quality for future generations. Effective measures must be implemented to prevent over-extraction, contamination from pollutants, and the degradation of aquifer systems. Integrated water resource management approaches, including monitoring and regulation, public awareness campaigns, and investment in sustainable technologies, are essential to safeguard this crucial resource.

Shallow fresh groundwater is prone to contamination from various anthropogenic surface activities, including agricultural runoff and industrial discharges. Deeper groundwater, which can be categorized into various types based on its characteristics and geological context, includes fossil groundwater, geothermal groundwater, and brine water. Fossil groundwater is ancient water that has been stored in aquifers for thousands of years and is typically not replenished under current climatic conditions [2]. Formation water, found in sedimentary rock pores since their formation, is often saline and located at significant depths. Geothermal groundwater is heated by the Earth’s interior and is valuable for energy production and heating. It is typically found in regions with tectonic activity [3]. Confined aquifers, while generally protected from surface pollution by the presence of impermeable rock or clay layers above them, are susceptible to a notable hazard known as upconing. Upconing is the upward migration of brackish or polluted water from the deeper strata of the groundwater system into freshwater zones within the confined aquifer. This phenomenon is commonly caused by excessive extraction or over-pumping of groundwater, resulting in a decline in aquifer pressure and facilitating the ascent of the underlying, more saline, water. Confined groundwater is found beneath impermeable layers of rock or clay, while unconfined groundwater is closer to the surface and interacts more readily with surface water sources. Confined aquifers are often more protected from contamination than unconfined aquifers due to impermeable layers of rock or clay that act as a barrier against pollutants by filtering the groundwater. However, it is important to note that confined aquifers can still be vulnerable to contamination from activities such as improper waste disposal or industrial spills that can breach the protective layers. Regular monitoring of water quality in confined aquifers is essential to detect early signs of contamination and prevent further degradation. Finally, brine water is characterized by its exceptionally high concentrations of dissolved salts. It is often a by-product of desalination processes and natural evaporation in arid regions and poses significant management challenges.

Thermal–mineral groundwater refers to water found in subsurface environments characterized by elevated temperatures and high concentrations of dissolved minerals. This type of water originates from deep geological processes where groundwater interacts with mineral-rich rocks and geothermal heat. It often has unique hydrochemical and isotopic properties due to prolonged water–rock interactions under high-pressure and high-temperature conditions [4]. Such water sources are typically found in the lower regions of the Earth’s crust and often ascend to the surface through preferential fault zones. Thermal–mineral groundwater mapping can be a valuable tool to identify potential sources of fresh groundwater pollution [5]. Furthermore, conducting periodic groundwater tracer tests can provide valuable insights into the movement and behavior of contaminants within confined aquifers, which allows the application of more effective mitigation strategies [6]. The distinctive configuration of thermal–mineral water exhibits variability based on the geological attributes of its source region, leading to a broad spectrum of mineral composition and potential therapeutic properties [7]. Thermal–mineral water may contain minerals like sulfur, magnesium, calcium, and silica, each presenting specific benefits for skin vitality, muscular relief, and holistic health [5]. The temperature of thermal–mineral groundwater is generally higher than that of normal groundwater because of geothermal heating. Waters can vary from moderately warm to exceptionally hot, frequently surpassing 50 °C (122 °F). Additionally, the pH of thermal–mineral groundwater can vary considerably, typically ranging from mildly acidic (around pH 6) to alkaline (reaching up to pH 9) [7]. Deeper aquifers are non-renewable groundwater resources where the rate of recharge is minimal compared to the extraction rate, thus making their use unsustainable [8]. Fossil water is typically characterized by its pristine quality, having been shielded from pollution and human activities for extensive periods of time, making it a valuable resource for regions facing water scarcity [9].

Formation water occurs naturally in oil and gas reservoirs and often contains high levels of salinity and various dissolved minerals. This water type originates from a variety of sources. It can be derived from ancient oceans that existed millions of years ago when rock formations were first deposited, and, as sediments accumulated and compacted over time, water was trapped within the rock pores and fractures. Formation water can also result from chemical reactions, such as mineral hydration or oxidation, which release water from their grid [10]. This water type is often encountered during oil and gas exploration and production activities and is typically found in reservoirs alongside petroleum deposits, occupying pore space in rock formations [11]. Formation water can have significant implications for oil and gas operations as it affects reservoir characteristics, such as pressure, permeability, and salinity. Understanding the chemical properties of formation water is essential to predict the locations of oil-rich reservoirs and favorable hydrocarbon accumulation areas, leading to the accurate identification of potential hydrocarbon reservoirs. These properties help determine the extent of hydrocarbon reserves, assess reservoir quality, plan drilling and production strategies, and manage fluid movement within reservoirs.

Connate water refers to water that was trapped in pore spaces in sedimentary rocks at the time of their early diagenesis. It is generally saline and has been cut off from the hydrologic cycle since its entrapment at the time of sedimentation. Unlike formation water, which may include fluids added or modified during diagenesis or hydrocarbon migration, connate water is considered to be the original interstitial rock water. Its composition reflects the depositional environment and can be highly diverse as a function of geological setting. Diagenesis processes and geochemical evolution of sedimentary basins, for which information is required, call for data on connate water [12].

Formation water samples are often collected and analyzed to determine their chemical composition, salinity, temperature, and other parameters. Formation water may help trace hydrocarbon mixing with thermal–mineral waters, offering a surface-level geochemical fingerprint. In this overview, we aim to reveal the processes involved and recorded case studies in the existing literature.

Brine is highly saline water that contains a high concentration of dissolved salts. The chemical composition of brine water often shows dominance of sodium and potassium ions, along with chloride and sulfate anions [13,14]. Additionally, brine water can have significantly higher total dissolved solids (TDSs) than freshwater, leading to super saturation for certain minerals like anhydrite. The salinity of brine water is significantly higher than that of seawater, often exceeding 35,000 parts per million (ppm) of dissolved salts [15].

The formation of brine water is primarily driven by geological and chemical processes such as water fixation by zeolite, chlorite formation, and mineral dissolution, as outlined in various research papers. Deep brines in contact with granites undergo reactions that lead to the concentration of residual salts, creating high salinity brines with specific mineral assemblages [16]. In arid and semi-arid regions, water bodies such as lakes, inland seas, and salt flats can become highly saline through the process of evaporation. As water evaporates, the concentration of dissolved salts increases, which leads to the formation of brine. This is common in places such as the Dead Sea, the Great Salt Lake, and salt pans [17]. Brine formation can be observed when groundwater encounters subterranean salt deposits, specifically halite. The interaction between water and salt leads to the dissolution of the latter, resulting in heightened salinity and the subsequent generation of brine. Naturally occurring brine formations are commonly found within geological structures rich in salt. Additionally, brine can be artificially produced through mining activities like solution mining. During this process, water is injected into salt deposits to dissolve the salt and generate brine, which is subsequently extracted to the surface via pumping mechanisms [18].

The relationship between brine and hydrocarbons is complex and impacts various processes in subsurface environments. As a common component in geological formations, brine interacts with hydrocarbons to influence phenomena like interfacial tension, diffusion coefficients, and storage capacities. Studies have shown that the presence of brine affects the diffusivity of hydrocarbons, with higher brine concentrations leading to reduced diffusion coefficients due to the formation of water clusters that affect hydrocarbon flows [19]. Additionally, the interfacial tension between brine and hydrocarbons is of utmost significance in determining pore-scale distribution and storage capacity, and prediction models have been developed to calculate this tension under different geo-storage conditions [20]. Furthermore, interactions between brine and hydrocarbons are essential in petroleum engineering as they affect the trapping of residual oil in reservoir pores [21]. Overall, knowledge of the interplay between brine and hydrocarbons is essential in understanding processes of CO₂ storage, energy generation, and ore formation in geological systems [22].

Groundwater pollution by hydrocarbons is a significant global environmental concern, influenced by both natural processes and human activities [23,24,25]. Originating from sources such as crude oil refineries and underground storage tanks, hydrocarbons can migrate into groundwater, impacting its quality and potentially posing risks to human health and ecosystems. The presence of hydrocarbons in groundwater alters bacterial communities, reducing diversity and favoring species tolerant to petroleum hydrocarbons, which can affect the functional potential for hydrocarbon degradation [26]. Interactions between water and rocks, driven by factors including salinity, influence the emission of hydrocarbons into groundwater, with high salinity promoting rock corrosion and hydrocarbon release [23]. Understanding the mechanisms of hydrocarbon emission into groundwater is crucial for assessing contamination levels, implementing effective monitoring strategies, and developing remediation technologies to safeguard groundwater resources and environmental health.

Human activities that contribute to groundwater contamination with hydrocarbons are diverse. These activities include petroleum refining, gas stations, and the improper disposal of hazardous wastes [25]. The production and refining of crude petroleum, as well as leakage from underground storage tanks at petrol stations, are major sources of hydrocarbon contamination in groundwater. This contamination poses health risks, particularly from compounds like benzene [23]. Moreover, the oil industry and gas stations are recognized as significant contributors to urban groundwater pollution on a global scale. This recognition underscores the need for continuous monitoring of groundwater quality to safeguard it and ensure its sustainable use [27]. Furthermore, the improper disposal of hazardous wastes, such as untreated wastes in landfills, can result in groundwater contamination through leachate.

The impact of hydrocarbon pollutants originating from petroleum industries and gas stations on urban groundwater quality is a significant concern, particularly with a more pronounced effect observed on shallow phreatic water compared to deeper confined water sources [28]. Moreover, the potential for wells, particularly in gas developments like coal seam gas (CSG) wells, to experience integrity failures can result in inter-aquifer leakage, ultimately leading to the potential depletion or contamination of groundwater reservoirs, particularly in instances involving historical coal exploration boreholes or reutilized gas wells as water extraction points [29]. Additionally, the presence of hydrocarbon contaminants in groundwater has been shown to bring about changes in bacterial populations, leading to a decrease in diversity while promoting the proliferation of specific species, thus impacting the functional capacity and breakdown of petroleum hydrocarbons within polluted areas [27]. These discoveries underscore the diverse range of influences that different hydrocarbons can exert on the quality and accessibility of groundwater resources, emphasizing the need for comprehensive understanding and management strategies in dealing with such environmental challenges.

Several natural mechanisms are accountable for producing natural hydrocarbon contamination of groundwater. These include seepage from petroleum-laden rocks and oil and gas migration through fractures in the Earth’s crust. Natural pollution by hydrocarbons may occur due to various factors, even including biogenic sources such as organic matter decomposition and biodegradation processes [30]. Hydrocarbons can naturally infiltrate groundwater through different mechanisms, including corrosion emission, extraction emission, and water–rock interactions. Research has demonstrated that hydrocarbons migrate from high potential strata to shallow groundwater in karst areas, impacting human health [31]. Furthermore, deep thermogenic gases, including methane, can migrate into aquifers through natural fractures, potentially contaminating groundwater sources [32]. Groundwater that contains hydrocarbon-degrading bacteria can enhance natural remediation as the microorganisms degrade heavy hydrocarbon molecules to less harmful substances through bioremediation processes [23].

In summary, groundwater, which serves as the primary source of fresh water for human consumption and ecosystem sustenance, is intricately connected to hydrocarbons, which encompass a wide range of organic compounds essential for energy/power production and industrial processes. These compounds may originate from natural sources such as crude oil and natural gas deposits or be a result of human activities such as industrial spills or improper disposal of hydrocarbon-based products.

A visual bibliographic network created using Connected Papers is depicted in Figure 1. The network is centered on the influential work by McMahon (2018) [33], which has become a key reference in studies exploring groundwater quality in hydrocarbon-rich regions. This paper appears as the most connected node, forming a conceptual nucleus around which a dense cluster of publications emerges. Closely associated studies such as those by Gillespie (2019 and 2022) [34,35], Stephens (2018 and 2021) [36,37], and Anders (2022) [38] reflect downstream applications and monitoring approaches, while more peripheral nodes (Vinson, 2013 [39], and Sice, 2018 [40]) address related topics with less citation overlap. The network also reveals a strong temporal concentration of research between 2016 and 2022, suggesting a period of intensified scientific attention on hydrogeochemical dynamics, oil and gas development, and groundwater protection. This visual analysis supports the identification of dominant research trends, emerging themes, and underexplored directions within the existing literature.

The primary aim of this literature overview is to critically analyze the current understanding of the interactions between hydrocarbons and thermal–mineral groundwater, with an emphasis on the methods employed to assess these interactions. The review attempts to provide an overview of current knowledge of the geochemical processes controlling hydrocarbon migration and their interactions with groundwater, with particular reference to thermal–mineral waters. With an emphasis on sustainability and environmental preservation, it also provides information on how hydrocarbon–groundwater interactions can support petroleum exploration and groundwater resource management. An overview of the interactions between groundwater and hydrocarbons has been notably absent from the existing literature. This overview provides all available articles highlighting current trends and future challenges, focusing on critical examination of the natural processes that lead to the co-existence of these valuable resources for humanity. Finally, it presents indicators that researchers can utilize to discern whether groundwater pollution, particularly concerning hydrocarbon compounds, arises from natural sources or anthropogenic activities.

The literature overview revealed a total of 39 field-site articles. The details of these are presented in Table S1, while Figure 2 shows the distribution of the case studies. The most significant literature on interactions between hydrocarbons and groundwater is summarized in Table 1.

This paper presents a conceptual subsurface model that presents the distribution and interaction of groundwater—particularly thermal–mineral water—and hydrocarbon reserves, including oil and gas (Figure 3). The lower section of the model shows the source rock, which contains organic content that can generate hydrocarbons where there is pressure and heat. Above this layer, permeable rock units such as carbonate and sandstone reservoirs serve as hydrocarbon storage sites due to their permeability. Over these lie impermeable shale sealing barriers that seal the hydrocarbons. Hydrocarbons can become trapped in some geological set-ups, i.e., trap-anticlines, trap-pinchouts, and trap-faults. Arch-shaped geological structures are anticlines, pinchouts are regions where porous reservoir layers taper and pinch out, and faults provide physical barriers against hydrocarbon migration. In addition, buoyant rock formations, salt domes, that rise up through overlying cover rocks tend to form good seals for oil and gas traps along their flanks. Buoyancy-induced migration routes of hydrocarbons are schematically shown ascending from the source rocks to the level where they are either cut off by seal rocks or contained by salt domes or the geometric traps of faults. The cross-section also indicates thermal–mineral water pathways cutting across the reservoirs and appearing at the surface as thermal–mineral springs. These waters have been subjected to widespread geological interaction, acquiring high temperatures and mineralization, while faults provide conduits for their upward movement. A layer of freshwater around the near-surface zone is represented, which indicates shallow groundwater systems typically bearing minimal mineralization and supporting human use and farming. Areas represented by the existence of hydrocarbons in conjunction with thermal–mineral waters indicate probable geochemical interactions and mixing processes with significant impacts on groundwater chemistry and hydrocarbon reservoir properties. Figure 3 emphasizes the complex subsurface dynamics and related functions of geological structures, groundwater flow, and hydrocarbon migration and storage. It clearly indicates that the diagram represents a conceptual deep subsurface model relevant to thermal–mineral water and hydrocarbon reservoirs. Shallow contamination pathways are not depicted in Figure 3 as they fall outside the primary scope of the current review.

2. Hydrocarbons in Geological Environments

Hydrocarbons are a class of organic compounds characterized by the covalent bonding between carbon and hydrogen atoms in various configurations. They play a significant role in organic chemistry as they are fundamental building blocks for essential substances, particularly fossil fuels like petroleum and natural gas. Hydrocarbons can be classified into different groups based on their structural characteristics: alkanes, alkenes, alkynes, and aromatics [41]. Alkanes, also known as paraffins, consist of saturated hydrocarbons with single carbon–carbon bonds. Alkenes have at least one carbon–carbon double bond, while alkynes have at least one carbon–carbon triple bond. Aromatics, like benzene, have a cyclic structure with delocalized pi electrons. These diverse hydrocarbon compounds are used in various industrial sectors, energy generation processes, and everyday applications [42].

Hydrocarbons such as petroleum (including crude oil and its derivatives), natural gas, and coal are recognized as viable sources for energy production. These hydrocarbons are valued for their substantial carbon content, which can be efficiently converted into heat or electricity through combustion processes. Petroleum extracted from underground reservoirs is the most widely used hydrocarbon for energy generation. It serves as the dominant fuel for transportation, heating, and power generation. Natural gas, primarily composed of methane, is another significant energy reserve utilized for heating, power production, and industrial applications. Additionally, coal, a solid hydrocarbon, has historically been a primary source of energy for electricity generation and industrial processes, although its usage has decreased in recent decades due to environmental concerns. Despite efforts to shift towards renewable energy alternatives, these hydrocarbons derived from the Earth’s crust remain indispensable for meeting global energy demands [43].

The natural origin of hydrocarbons involves the decomposition of lipids or oils combined with specific plant materials, and the hydrocarbons are obtained as by-products of distillation, such as crude oils, gasoline, kerosene, diesel, heavy oils, or asphalts. They can exist in the environment as aliphatic or aromatic compounds. Aliphatic hydrocarbons originate from lipids or oils, while aromatic hydrocarbons originate from specific botanical sources. Hydrocarbons are one of the most common environmental contaminants detected in groundwater. Their ability to migrate from geological formations with high hydrocarbon potential to shallow groundwater is influenced by corrosion and emissions relating to extraction activities [44].

Hydrocarbons migrate within geological formations during petroleum generation, a process driven by thermal maturation acting on organic matter. Organic matter, which includes proteins, carbohydrates, lipids, and lignins, transforms into kerogen through burial diagenesis [45]. Kerogen formation occurs in shallow subsurface layers through the biogenic breakdown of organic matter, resulting in the release of methane, carbon dioxide and water, and the formation of a complex hydrocarbon compound known as kerogen [46]. Subsequently, during catagenesis in the deeper subsurface, kerogen releases petroleum. Primary migration involves the transfer of hydrocarbons from the source rock to permeable carrier beds, followed by secondary migration, which includes the subsequent movement of oil and gas within these beds and reservoirs [47]. Migration pathways may include lateral contact, source rock–reservoir contact, fault-connected routes, and source rock–reservoir superposition. The migration process can be observed and studied through experimental simulations such as sandbox models and industrial CT scanning. Seismic imaging techniques can further provide insights into the subsurface structure of gas-charged sediments, assisting in the identification of hydrocarbon migration pathways.

Hydrogeology plays a pivotal role in controlling the movement of hydrocarbons within aquifers. Research indicates that hydrogeochemical markers, such as anomalies characteristic of oil fields, are important when evaluating the oil contents of aquifers [48,49]. Moreover, the presence of faults within Paleozoic formations has been recognized as a significant factor that aids the transportation of hydrocarbon-rich brines from deeper Devonian source rocks to overlying aquifers, consequently impacting the geochemistry of shallow groundwater within oil-producing regions [50]. Additionally, hydrochemical conditions within fractured karst aquifers can undergo substantial alterations due to the presence of petroleum hydrocarbons, resulting in modifications in hydrochemistry patterns and the evolutionary processes of the aquifer over an extended time period. It is vital to comprehend these hydrogeological influences to effectively manage and safeguard groundwater reservoirs against potential hydrocarbon contamination [24]. Understanding these dynamics can help ensure sustainable utilization of groundwater resources and minimize the risk of pollution from hydrocarbons. The exploration of these interactions provides valuable insights for all policymakers, researchers, and stakeholders involved in the preservation and management of water resources in hydrocarbon-rich areas.

Aquifers possessing diverse characteristics, including heterogeneity, anisotropy, and permeability, have the potential to exert a profound influence on the flow and distribution of hydrocarbons within the subsurface realm [51]. The presence of faults within Paleozoic formations, for example, emerges as a pivotal element that aids in the conveyance of hydrocarbon-laden brines from deeper Devonian reservoirs into the overlaying aquifer strata, underscoring the significance of geological attributes in governing the migration patterns of hydrocarbons. Moreover, research has pointed out that the heterogeneity of aquifers, characterized by abrupt variations in lithology and permeability across short vertical intervals, can significantly alter the migration pathways and behavior of pollutants such as chlorinated hydrocarbons, thus highlighting the need for thorough comprehension of aquifer structures to ensure precise elucidation of hydrocarbon transport phenomena [52].

Porosity plays an important role in influencing hydrocarbon flow within aquifers, as evidenced by studies highlighting the substantial impact of vugs and interconnected pore spaces on hydrocarbon flow dynamics in carbonate aquifers. The presence of porous and permeable flow units characterized by interconnected vugs is instrumental in facilitating the movement of fluids, as indicated by research findings [53]. Moreover, the porosity of the rock formation exerts a notable influence on compressional- and shear-wave velocities, with lower P-wave velocities being associated with higher porosity levels, thereby suggesting a heightened potential for fluid flow [54]. The management of hydrocarbon aquifers highlights the critical nature of comprehending porous media and the fluids they contain to ensure operational safety and regulatory adherence [51]. Beyond its role in hydrocarbon flow, porosity also governs the efficiency of heat transfer in aquifers used for geothermal energy purposes, where changes in porosity levels can significantly impact thermal transport mechanisms and energy extraction rates. In essence, porosity emerges as a fundamental parameter with multifaceted implications for fluid flow dynamics, heat transfer processes, and overall operational considerations within aquifer systems [55].

Aquifer structures have different impacts on the flow of hydrocarbons. Fault zones can act as barriers to fluid movement and cause disruptions in aquifer potentiometric surfaces. Tracing the movement of hydrocarbons can be aided by fine-grained authigenic carbonates with negative carbon isotopic signatures, which can help map migration pathways and determine flow speeds [56]. Parameters like density differences, interfacial tension, and wettability affect the distribution and movement of hydrocarbons in porous media, thus influencing their movement in aquifers [57]. Geophysical methods such as electrical resistivity tomography and ground-penetrating radar can be used to monitor and characterize gas migration in aquifers. Factors such as heterogeneity, anisotropy, and groundwater velocity can all impact the behavior of gas plumes [51].

The potential contamination of groundwater from conventional and unconventional oil and gas hydrocarbon extraction sites has become a major issue in recent years. Conventional hydrocarbons refer to those that can be easily explored and extracted using traditional methods, while unconventional hydrocarbons are more challenging to access due to their unique geological characteristics and require specialized extraction techniques [58]. Unconventional hydrocarbons include resources such as gas hydrates, tar sands, tight oil reservoirs, oil shales, shale gas, and coal bed methane, which have distinct source rock–reservoir rock-trap characteristics and necessitate extraction methods such as surface mining, retorting, subsurface heating, and hydraulic fracturing [59]. These unconventional resources offer vast reserves globally, potentially exceeding the energy content of conventional oil reserves [60]. The extraction of hydrocarbons from unconventional reservoirs often involves directional drilling and reservoir stimulation techniques including hydraulic fracturing to enhance production from tight source rocks [29].

Conventional and unconventional methods of oil and gas extraction present notable environmental challenges, such as water contamination, deterioration of air quality, destruction of habitats, and the release of greenhouse gases [61,62]. Unconventional techniques, for example, hydraulic fracturing, necessitate substantial volumes of water, giving rise to concerns regarding pollution and the management of wastewaters. Similarly, conventional practices also play a role in water resource challenges due to the disposal and injection of produced water [63].

Salt structures play a crucial role in influencing the movement of fluids, such as oil, in aquifers that are not uniformly mixed. The presence of fractures in deep saline aquifers further adds to aquifer heterogeneity, affecting the transport of substances like carbon dioxide (CO₂) [46]. Additionally, the size and arrangement of different geological features have a significant impact on the distribution of saltwater and its circulation in coastal aquifers. These factors have implications for predicting submarine groundwater discharge (SGD) [64]. The displacement of fluids with varying densities and viscosities, such as oil, is a basic issue in confined aquifers with practical applications in scenarios such as oil recovery and CO₂ sequestration in geological formations [65].

Salt structures influence oil migration in aquifers by creating complex pathways and deformation patterns in surrounding strata, impacting the migration of fluids. The movement of salt within the subsurface can lead to the development of salt diapirism affecting geological sensitivities and the evolution of salt structures. Additionally, the presence of fractures in geological structures, such as deep saline aquifers, enhances aquifer heterogeneity and influences the migration of substances like CO₂. Fractures serve as preferred channels for migration and can alter migration rates depending on their length, aperture, and orientation [66]. These structures act as cap rocks, creating large traps and maintaining high porosity, facilitating the migration and accumulation of oil and gas in basins such as the La Popa Basin in northeastern Mexico [67]. Salt-related structures like salt pillows and salt walls provide favorable spaces for hydrocarbon accumulation, while faults serve as pathways for migration, and thick halite layers act as regional seals that preserve hydrocarbons. Additionally, various salt structures contribute significantly to the migration and accumulation of oil and gas, aiding the formation of large oil and gas fields. The role of tectonic activity in shaping the sedimentation and re-sedimentation processes of the Messinian evaporites provides valuable insights into their distribution and potential as seal rocks in hydrocarbon exploration [68].

The presence of evaporite layers, such as anhydrite and halite, affects the depositional environment and source rock potential for petroleum generation. Evaporites create different environments within a basin, with highly saline conditions favoring the formation of evaporite layers and brackish to fresh-water conditions influencing the upper layers where petroleum source rock potential is best. Additionally, the thermal properties of evaporites impact the thermal evolution of organic matter, accelerating hydrocarbon generation in shallower depths where evaporites are present and widening the hydrocarbon generation window in deeper strata under evaporite sequences [66]. This phenomenon is common in evaporite-bearing basins and highlights the significance of understanding evaporite deposition and its influence on oil migration and accumulation in sedimentary basins. Evaporites control oil migration by acting as cap rocks, maintaining temperature balance, creating reservoir fractures, and forming traps, impacting the formation and distribution of giant oil fields globally [69]. Moreover, evaporites control oil accumulation by providing top seals, lateral seals, and traps in sedimentary basins during lowstands, influencing the distribution of hydrocarbons in stratigraphic sections [70].

The presence of salty groundwater can potentially serve as a proxy indicator for the presence of oil in each region, as recorded in various studies. Deep aquifers with high salinity levels have been identified in oil fields such as the Barmer Basin in India, where saline water abstraction is crucial for oil field operations [71]. Furthermore, geoelectric resistivity surveys in the Issaran oil field in Egypt have shown the presence of saline water-bearing horizons structurally controlled by geological elements, potentially indicating pathways for seawater intrusion into the oil field area [72]. These findings collectively suggest that monitoring the salinity of groundwater can provide valuable insights into the presence of oil in a region [73].

3. Groundwater Interaction with the Hydrocarbon Reservoir

Hydrocarbons can naturally enter groundwater sources through various mechanisms, including the degradation of fats, oils, and plant extracts, as well as the production and refining of crude petroleum and leaking underground storage tanks at petrol stations [74]. Once hydrocarbons are released, they undergo physical, chemical, and biological changes [32]. The degradation of hydrocarbons depends on their properties. Gravity influences the downward migration of hydrocarbons through the unsaturated zone. In soil, hydrocarbons can exist in different phases, such as dissolved in water, sorbed on solid particles, in soil gas, and as free-phase liquids [23]. The presence of hydrocarbon contamination in groundwater can be confirmed by analyzing redox-sensitive compounds. These compounds indicate the range of redox processes, from strictly anoxic to aerobic conditions. Different biodegradation mechanisms contribute to the attenuation of hydrocarbons in groundwater [75].

The main sources of hydrocarbons in groundwater are predominantly anthropogenic, stemming from a variety of human activities such as petroleum refining, oil depots, chemical plants, industrial processes, transportation, heating, and the combustion of fossil fuels such as coal [23,76]. These activities result in the release of hydrocarbons into the environment, where they can infiltrate the soil and migrate downwards, ultimately leading to groundwater contamination. Hydrocarbons, including both aliphatic and aromatic compounds, are prevalent contaminants in groundwater due to their presence in crude oils, gasoline, diesel, and other petroleum products and pose a significant concern for both groundwater quality and human health [77]. Therefore, it is crucial to implement effective monitoring and management strategies for these pollution sources in order to safeguard groundwater resources and ensure the provision of safe drinking water for communities [78].

Hydrocarbons, including BTEX compounds (Benzene, Toluene, Ethylbenzene, Xylenes), can percolate through soil layers based on their physical and chemical properties, with their movement influenced by factors such as soil texture and permeability [23]. Once hydrocarbons enter the soil, they can migrate downwards through the unsaturated zone under the influence of gravity [44]. The presence of hydrocarbons in groundwater poses significant risks to human health and stresses the importance of comprehending the mechanisms and sources of hydrocarbon contamination to implement effective mitigation measures. Table 1 presents a summary of the most important research papers on hydrocarbon and groundwater interactions in the current bibliography.

Hydrocarbons have the potential to infiltrate karst areas via multiple pathways. Within karst underground river systems, the movement of slow-flowing water through karst fissures and fast-flowing water through conduits facilitates the transport of polycyclic aromatic hydrocarbons (PAHs) from the surface to the subsurface during rainfall events [79]. In addition, interactions between water and rock in karst regions can result in the release of hydrocarbons into groundwater, with the salinity factor playing a significant role in driving hydrocarbon migration [78]. In the Buda Hills (Hungary), the migration of hydrocarbons within fractures filled with minerals, such as calcite, barite, and sulfides, suggests that hydrocarbons have migrated concurrently with mineralizing fluids. It is likely that hydrocarbon-bearing fluids have moved in a north-westward direction from the eastern basin to the hills since the Miocene period [80]. Pollution sources such as refineries, oil depots, chemical plants, and mechanical factories significantly contribute to the contamination of groundwater with petroleum hydrocarbons, making it a noteworthy source of pollution in karst areas [27]. In a heavy industry district located in southwestern China, it was discovered that the PAHs recorded in the local karst groundwater originated from coal burning and industrial activities. The karst aquifers in this region are severely affected by pollution due to favorable migration conditions and reduced self-purification capabilities [53].

4. Methods of Identifying Hydrocarbons in Groundwater

Hydrocarbons have a significant impact on the quality and availability of groundwater resources as they cause contamination and pollution. Research conducted in Nigeria has demonstrated that hydrocarbon spills from petroleum depots and gas stations have resulted in widespread groundwater contamination, with shallow aquifers being more severely affected than deep ones [25,28]. The deterioration of groundwater quality due to hydrocarbons is of global concern, with contaminants such as nitrites and nitrates originating from human activities posing serious threats to water quality [23]. Moreover, microbial communities in sites contaminated with petroleum hydrocarbons are noteworthy in biogeochemical processes, which affect both groundwater quality and bacterial diversity [26]. In addition, methane leakage from hydrocarbon wellbores can impair the quality of groundwater, indicating the importance of maintaining wellbore integrity to prevent methane migration and subsequent contamination [81]. Consequently, efforts directed towards enhancing our knowledge of the interactions between hydrocarbons and groundwater are essential for developing robust management practices that can effectively safeguard this vital resource from pollution.

The composition of ions in formation water significantly influences hydrocarbon accumulation and preservation and ions affect hydrocarbon composition. For instance, the presence of Mg²⁺ ions in formation water acts as a catalyst, promoting crude oil cracking to gas and altering the ratios of different gas components [82]. Additionally, the geochemical characteristics of formation water, such as salinity levels and specific ion ratios, are closely related to gas preservation and can indicate favorable areas of hydrocarbon accumulation in different reservoir types, including carbonate and tight sandstone reservoirs [83]. The geochemical interactions between formation water ions and hydrocarbons are significant for evaluating hydrocarbon resources in deep sedimentary basins and predicting hydrocarbon dynamics accurately.

When hydrocarbons infiltrate aquifers, they can compromise the quality of groundwater, rendering it unsuitable for consumption and threatening dependent ecosystems dependent [84]. The movement of hydrocarbons within groundwater systems can have far-reaching consequences, potentially impacting larger areas and necessitating complex remediation efforts to restore water quality. Comprehensive geochemical data suggest that saline groundwater may have originated through natural processes, potentially from the migration of deeper methane-rich brines that have interacted extensively with coal lithologies [85].

The interaction between groundwater and hydrocarbons involves the volatilization and solubility characteristics of hydrocarbon components in air, soil, and water compartments. The transport characteristics of hydrocarbons in the porous matrix and residual liquid are influenced by changes in the viscosity and density of the non-aqueous liquid [86]. Groundwater can become contaminated with hydrocarbons due to petroleum spills and industrial activities, leading to qualitative degradation of the groundwater [87]. The extent of groundwater contamination is influenced by factors such as the water table depth, soil permeability, and infiltration rate [88]. Monitoring and investigating the quality and contamination level of groundwater is essential for groundwater extraction, usage, and protection, especially in areas with petroleum and petrochemical production [25]. Urban groundwater pollution caused by the oil industry and gas stations is a significant global environmental problem, with shallow groundwater being more severely contaminated than deep groundwater. These interactions highlight the importance of monitoring and investigating groundwater quality and contamination levels in order to protect this vital resource.

The presence of petroleum and petrochemical industries can lead to groundwater pollution, with hydrocarbons being a significant source of contamination [25]. The quality of groundwater is impacted by factors such as total dissolved solids, total hardness, and specific ions like Cl⁻ and SO₄²⁻, which can be influenced by both natural processes and human activities [88]. Water also plays a crucial role in reducing hydrocarbon permeability and interrupting flow through rocks of low porosity and permeability, impeding flow or trapping some gases [89]. As hydrocarbons are transported through the subsurface, water affects the fluid flow, heat transfer, and mass transfer, and their concentration in water can be revealed graphically. The interactions between water and hydrocarbons can impact the migration and accumulation process of the latter in tight formations [90].

Chromatography, specifically gas chromatography mass spectrometer (GC–MS) and gas chromatography with flame ionization detector (GC–FID), can effectively identify the source of hydrocarbon contamination in groundwater samples [91]. These techniques allow the qualitative and quantitative identification of hydrocarbon compounds, including polycyclic aromatic hydrocarbons (PAHs), which are highly toxic and carcinogenic [92]. Chromatography is a valuable technique for identifying the source of hydrocarbon contamination in groundwater. The process commences with the collection and preparation of groundwater samples, from which hydrocarbons are extracted from the solution. gas chromatography (GC), often paired with mass spectrometry (GC–MS), is the preferred method for analyzing these hydrocarbons due to its effectiveness in separating and identifying complex mixtures based on their volatility and molecular characteristics [93]. The produced chromatograms and mass spectra yield detailed insights into the hydrocarbon composition, facilitating the identification of specific compounds in the samples. To determine the origin of contamination, chromatographic patterns, or fingerprints, of the hydrocarbons in the groundwater are compared to those of suspected sources, including crude oil, petroleum products, and industrial discharges. This comparison includes examining specific hydrocarbon ratios and unique marker compounds associated with different sources.

Furthermore, isotopic analysis of stable carbon or hydrogen isotopes in the hydrocarbons can provide additional information about the contamination’s source. By combining these analytical techniques, chromatography effectively identifies the source of hydrocarbon contamination, which is essential for remediation efforts and legal inquiries [94].

Chromatography, particularly liquid chromatography with organic carbon detection and organic carbon isotope geochemical analysis, is necessary to differentiate hydrocarbon sources in groundwater [95]. This method enables the identification of dissolved organic carbon (DOC) sources based on their concentration, carbon isotopic composition, and other characteristics, such as distance from surface water sources, depth below the surface, and groundwater residence time. Additionally, two-dimensional gas chromatography with time-of-flight mass spectrometry helps tentatively identify different oxygen-containing organic compounds in groundwater, providing insights into the presence of petroleum hydrocarbons and their degradation intermediates [96]. The combination of gas chromatography–mass spectrometry and liquid–liquid extraction allows for the simultaneous analysis of various polycyclic aromatic hydrocarbons (PAHs) in groundwater, facilitating the identification and quantification of different PAHs and their derivatives to assess groundwater pollution levels [97].

GC–MS is generally more effective than fluorescence spectroscopy for the analysis of complicated mixtures, unknown substances, and the detection of non-fluorescing analytes. GC–MS is more appropriate to analyze and identify the separate components of complicated samples and provide a molecular fingerprint in terms of mass spectra, which proves particularly beneficial for complicated environmental samples or for forensic purposes. In contrast, fluorescence spectroscopy is limited to analytes that incidentally fluoresce or which can be derivatized to fluoresce and is, therefore, less universal for non-fluorescent analytes. Moreover, GC–MS possesses good specificity and sensitivity that enable the identification of trace levels of analytes even in the presence of interferants, while fluorescence spectroscopy can be interfered with by sample matrix background fluorescence or overlapping emission spectra. Therefore, for comprehensive qualitative and quantitative analysis, especially for complex or unknown samples, GC–MS is usually the method of choice.

Hydrocarbon forensics is one of the major and emerging sciences in environmental hydrogeology and is applied to detect, discriminate, and trace sources of hydrocarbon groundwater contamination. Hydrocarbon forensics combines chemical, isotopic, and geospatial analytical methods to discriminate between naturally occurring and anthropogenically derived hydrocarbons such as fuel leaks, refinery seep, or industrial discharge [98]. The most utilized technique is compound-specific isotope analysis (CSIA), which investigates the isotopic fingerprint (e.g., δ¹³C and δ²H) of individual hydrocarbon compounds to determine their origin and biodegradation pathway. CSIA can determine biodegradation phases and discriminate between similar contaminants from various sources. In parallel, hydrocarbon fingerprinting uses diagnostic ratios of aliphatic and aromatic hydrocarbons (e.g., n-C17/Pristane and PAH profiles) to provide confident source attribution even in the presence of complex multi-source contamination patterns [99].

Hydrocarbon forensics has been supplemented recently by new developments in statistical and spatial modeling. Multivariate techniques such as principal component analysis (PCA) and hierarchical clustering allow the detection of patterns within large datasets that can aid source apportionment and detection of degradation patterns. These methods combined with GIS-based visualization software (ArcGis Pro 3.5) can generate effective hydrocarbon plume maps, locate potential points of release, and reconstruct contaminant pathways [100]. Forensics and time-series monitoring can also help determine if the pollution is legacy, active, or recent by tracking temporal evolution in contaminant composition. These tools are a complete system of evidence-based decision-making in contamination science, site clean-up, and legal allocation of faults. The use of such forensic strategies in groundwater–hydrocarbon studies significantly increases the understanding of contaminant fate and the efficacy of environmental management techniques.

Table 1. Summary of the major literature reviews on hydrocarbon and groundwater interactions.

A/A	Reference	Year	Location	Methodology	Aquifer Type
1	Warner et al. [101]	2012	Pennsylvania	Geochemical data of produced water (PW) samples/processes that control the quality of PW generated from hydrocarbon bearing formations by analyzing relationships between their major ion concentrations.	Shallow
2	Peterman Z. et al. [102]	2012	Northeastern Montana, USA	Analyzing water quality parameters before and after petroleum hydrocarbon pollution. Studying hydrogeochemical mechanisms including desulfurization, denigration, and ion exchange processes.	Deep (brine)
3	Li H. et al. [103]	2017	Colorado, USA	Isotope mixing model based on δ2H and δ18O. Cl mixing model for PW contribution assessment. Validation of Na, Cl, Br, Sr, Ba, Li, and B tracers.	Confined and shallow
4	Wang Q. et al. [104]	2020	NW China	Using hydrochemical indicators (Ca2+, Mg2+, Na+, K+, HCO3−, NO3−, Cl−, F−, and SO42−) and pH with the help of GIS and origin platforms, statistical analyses, and graphical methods.	Confined and shallow
5	Apango F. et al. [105]	2021	Mexico	Inorganic geochemistry (major cations and anions). Stable isotopes of select inorganic constituents (Sr, B, Li, and C). Select hydrocarbon molecular and isotopic tracers. Tritium and noble gas elemental and isotopic composition.	Deep/confined
6	McMahon P. et al. [106]	2021	USA	Hydrochemistry and water type (HCO3-Ca), δ13C (−16.1 to −11.7‰) indicating the equilibrium fractionation between CO2-DIC. Based on the analysis above and published; other shale gas production projects, TDS; major elements and stable isotopes of δ13CDIC can be chosen as inorganic geochemical monitoring indicators.	Confined and shallow
7	Rosecrans Z.C. et al. [107]	2021	California, USA	Chemical and isotopic tracers were analyzed in groundwater samples to assess the presence of thermogenic gas or water from hydrocarbon-bearing formations mixing with surrounding groundwater. Investigating the occurrence of light hydrocarbon gases in groundwater.	Confined and shallow
8	Stephens M. et al. [37]	2021	California, USA	Produced water TDS samples were utilized for model construction. Borehole geophysics data were incorporated into the TDS model. The study analyzed stratigraphy and structure impact on TDS.	Confined and shallow
9	Yang Z. et al. [83]	2022	Shandong Province, N China	Analysis of geochemical composition of formation water. Gini coefficient used for weighted calculation of parameters. Decision tree model applied for correlation analysis.	Deep/formation water
10	Gillespie J. et al. [35]	2022	California, USA	Geophysical log analysis maps groundwater salinity distribution. Data includes formation picks and water quality information. Log-calculated TDS used in TDS versus depth plots.	Deep aquifer
11	McMahon P. et al. [108]	2023	California, USA	Sampling of groundwater wells, springs, or seeps. Isotopic and gas compositional analysis. Standard methods for analyzing water and gas components. Collection of replicate and single sample wells.	Confined and shallow
12	Warden J. et al. [109]	2024	California, USA	The collected samples were analyzed for a broad suite of constituents, including major and minor ions, nutrients, trace elements, stable isotopes of water, groundwater-age tracers, noble gases, hydrocarbon gases, and volatile organic compounds (VOCs).	Deep aquifer
13	McMahon P. et al. [110]	2024	California, USA	Groundwater was sampled laterally and vertically within the aquifer for multiple geochemical tracers. This comprehensive sampling approach helped reveal the complex interactions affecting groundwater quality.	Confined and shallow
14	Zhang H. et al. [111]	2024	Northwest China	Absorbance measurements, DOC concentrations from oil field samples ranged widely from 68 mg/L to nearly 3000 mg/L, BTEX concentration. PCA analysis.	Confined and shallow
15	Rusi S. et al. [112]	2018	Apennine, Italy	Conducted over 6 years (2011–2017) with multiple groundwater sampling campaigns in the Gran Sasso aquifer system. Samples were analyzed for dissolved hydrocarbons, geochemical tracers, and isotopic analyses (δ13C of methane and DIC).	Carbonate/karst
16	Schloemer J. et al. [113]	2016	L. Saxony, Germany	Focus was on dissolved methane (CH4), ethane (C2H6), and propane (C3H8) concentrations. Accompanied by measurements of δ13C–CH4 and δ2H–CH4 for source differentiation.	Shallow

Table 1. Summary of the major literature reviews on hydrocarbon and groundwater interactions.

A/A	Reference	Year	Location	Methodology	Aquifer Type
1	Warner et al. [101]	2012	Pennsylvania	Geochemical data of produced water (PW) samples/processes that control the quality of PW generated from hydrocarbon bearing formations by analyzing relationships between their major ion concentrations.	Shallow
2	Peterman Z. et al. [102]	2012	Northeastern Montana, USA	Analyzing water quality parameters before and after petroleum hydrocarbon pollution. Studying hydrogeochemical mechanisms including desulfurization, denigration, and ion exchange processes.	Deep (brine)
3	Li H. et al. [103]	2017	Colorado, USA	Isotope mixing model based on δ2H and δ18O. Cl mixing model for PW contribution assessment. Validation of Na, Cl, Br, Sr, Ba, Li, and B tracers.	Confined and shallow
4	Wang Q. et al. [104]	2020	NW China	Using hydrochemical indicators (Ca2+, Mg2+, Na+, K+, HCO3−, NO3−, Cl−, F−, and SO42−) and pH with the help of GIS and origin platforms, statistical analyses, and graphical methods.	Confined and shallow
5	Apango F. et al. [105]	2021	Mexico	Inorganic geochemistry (major cations and anions). Stable isotopes of select inorganic constituents (Sr, B, Li, and C). Select hydrocarbon molecular and isotopic tracers. Tritium and noble gas elemental and isotopic composition.	Deep/confined
6	McMahon P. et al. [106]	2021	USA	Hydrochemistry and water type (HCO3-Ca), δ13C (−16.1 to −11.7‰) indicating the equilibrium fractionation between CO2-DIC. Based on the analysis above and published; other shale gas production projects, TDS; major elements and stable isotopes of δ13CDIC can be chosen as inorganic geochemical monitoring indicators.	Confined and shallow
7	Rosecrans Z.C. et al. [107]	2021	California, USA	Chemical and isotopic tracers were analyzed in groundwater samples to assess the presence of thermogenic gas or water from hydrocarbon-bearing formations mixing with surrounding groundwater. Investigating the occurrence of light hydrocarbon gases in groundwater.	Confined and shallow
8	Stephens M. et al. [37]	2021	California, USA	Produced water TDS samples were utilized for model construction. Borehole geophysics data were incorporated into the TDS model. The study analyzed stratigraphy and structure impact on TDS.	Confined and shallow
9	Yang Z. et al. [83]	2022	Shandong Province, N China	Analysis of geochemical composition of formation water. Gini coefficient used for weighted calculation of parameters. Decision tree model applied for correlation analysis.	Deep/formation water
10	Gillespie J. et al. [35]	2022	California, USA	Geophysical log analysis maps groundwater salinity distribution. Data includes formation picks and water quality information. Log-calculated TDS used in TDS versus depth plots.	Deep aquifer
11	McMahon P. et al. [108]	2023	California, USA	Sampling of groundwater wells, springs, or seeps. Isotopic and gas compositional analysis. Standard methods for analyzing water and gas components. Collection of replicate and single sample wells.	Confined and shallow
12	Warden J. et al. [109]	2024	California, USA	The collected samples were analyzed for a broad suite of constituents, including major and minor ions, nutrients, trace elements, stable isotopes of water, groundwater-age tracers, noble gases, hydrocarbon gases, and volatile organic compounds (VOCs).	Deep aquifer
13	McMahon P. et al. [110]	2024	California, USA	Groundwater was sampled laterally and vertically within the aquifer for multiple geochemical tracers. This comprehensive sampling approach helped reveal the complex interactions affecting groundwater quality.	Confined and shallow
14	Zhang H. et al. [111]	2024	Northwest China	Absorbance measurements, DOC concentrations from oil field samples ranged widely from 68 mg/L to nearly 3000 mg/L, BTEX concentration. PCA analysis.	Confined and shallow
15	Rusi S. et al. [112]	2018	Apennine, Italy	Conducted over 6 years (2011–2017) with multiple groundwater sampling campaigns in the Gran Sasso aquifer system. Samples were analyzed for dissolved hydrocarbons, geochemical tracers, and isotopic analyses (δ13C of methane and DIC).	Carbonate/karst
16	Schloemer J. et al. [113]	2016	L. Saxony, Germany	Focus was on dissolved methane (CH4), ethane (C2H6), and propane (C3H8) concentrations. Accompanied by measurements of δ13C–CH4 and δ2H–CH4 for source differentiation.	Shallow

Table 1 summarizes key literature that has contributed significantly to our understanding of the hydrocarbon–groundwater relationship, particularly in thermal–mineral and confined aquifer systems. The research encompasses a range of approaches from isotope tracing [103] to hydrochemical indicators [104] and using noble gases and isotopic compositions [106]. These techniques have made important contributions to the detection of the occurrence and movement of hydrocarbons in shallow and deep aquifers and discrimination between natural seepage and anthropogenic pollution.

For example, Warner et al. (2012) [101] offered early evidence of water quality changes associated with unconventional gas development in Pennsylvania, USA, with a focus on the contributions of major ion changes and formation water inputs. Similarly, Peterman et al. (2012) [102] in Montana, USA, investigated hydrogeochemical processes such as desulfurization and ion exchange and explained the long-term brine-impacted aquifer development. These earlier studies, though sometimes under-represented in recent reviews, contain valuable information on baseline conditions and processes of pollutant transformation.

More recent studies, such as Rosecrans et al. (2021) [107] and Zhang et al. (2024) [111], incorporate newer analytical techniques (e.g., stable isotope fingerprinting, DOC, and BTEX quantification) to more robustly confine hydrocarbon contaminant sources. The time span of the studies, from early 2010s seminal publications to present state-of-the-art research, provides an overall perspective of the methods and hydrogeochemical indicators defining groundwater–hydrocarbon interactions in diverse geological settings.

Hydrocarbons intrude groundwater systems naturally through interactions with organic-rich rock units such as bituminous dolomites and limestones that are petroleum-charged. For instance, studies in the Central Apennines of Italy have established that natural leaching from bituminous rocks in carbonate aquifers can produce traceable levels of hydrocarbons in springs, even in the absence of anthropogenic contamination (Rusi et al., 2018) [112]. This helps to emphasize the requirement of distinguishing between the natural background concentrations of hydrocarbons and contamination by oil field operations or spill incidents. In hydrocarbon-producing areas, like the Poso Creek Oil Field of California, groundwater contamination with hydrocarbons (benzene and methane) can be due to complex interactions between land use, infrastructure, and natural hydrogeologic processes. Groundwater in these settings will flow through aquifers that are in contact with oil-bearing reservoirs in which structural features like faults or badly sealed well casing can serve as pathways for hydrocarbon migration. Nevertheless, natural processes such as sulfate reduction also contribute to the breakdown of the hydrocarbons in the aquifer to limit their extent. Therefore, evaluating the existence of hydrocarbons in groundwater needs to be founded on a multidisciplinary approach integrating hydrology, geochemistry, isotopes, and geology to discern the probable difference between natural and anthropogenic sources (McMahon et al., 2024) [110]. The connection between groundwater and hydrocarbon reservoirs may be multiple and influenced by natural and anthropogenic factors, particularly in regions with high oil deposits. Groundwater systems in the Niger Delta, for example, are typically threatened by hydrocarbon pollution via oil exploration, pipeline leaks, and artisanal refining. The widespread occurrence of volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and xylene (BTEX) in groundwater has been documented, which has been found to have severe impacts on human health. In addition to direct pollution, hydrocarbons that migrate to groundwater can undergo biodegradation, a natural process, which impacts their transport and fate in aquifers. Techniques such as CO₂ isotopic tracking have been utilized to estimate in situ biodegradation rates in contaminated groundwaters and offer insights on the effectiveness of natural attenuation (Guimbaud et al., 2023) [114]. It is essential to quantify these interactions not just to monitor the environment but also to design effective remediation strategies, especially in regions where groundwater is a significant source of drinking water.

5. Characteristics of Thermal–Mineral Water

Thermal–mineral groundwater is characterized by elevated temperatures exceeding the average water temperature of the surrounding area and high concentrations of dissolved minerals. It is commonly found in geothermal regions and areas with significant subsurface heat flow. This unique type of groundwater forms through prolonged interactions between water and rocks at elevated temperatures. These interactions result in a distinct chemical composition enriched with various minerals and trace elements. When activities related to the extraction of hydrocarbons, such as drilling and hydraulic fracturing, take place in regions where thermal–mineral groundwater is present, significant interactions may occur. These extraction activities can disrupt natural flow patterns and pressure conditions, thereby posing a contamination risk through the introduction of hydrocarbons and fluids utilized during fracking operations. Contamination can have adverse effects on water quality, reducing its suitability for applications like geothermal energy production, spa treatments, or other uses. Furthermore, the high temperatures and unique chemical make-up of thermal–mineral groundwater can impact the behavior of hydrocarbons, adding complexity to the extraction process. To tackle these challenges, effective management strategies and rigorous environmental protection measures must be implemented. This is essential in order to mitigate risks and guarantee the sustainable coexistence of hydrocarbon extraction activities and the utilization of thermal–mineral groundwaters.

Hydrocarbon extraction, which includes tasks such as drilling, hydraulic fracturing, and the management of produced water, has sparked considerable environmental alarms, especially in terms of its effects on water resources. Thermal–mineral waters are of particular concern due to their distinctive chemical characteristics and wide-ranging applications, such as geothermal energy generation, therapeutic purposes, and mineral extraction.

Table 2. Range of main elements present in different water types.

Water Type/ Elements (mg/L)	Fresh Water	Thermal Water	Formation Water	Brine Water
Chloride (Cl−)	1–10	50–5000	10,000–200,000	50,000–300,000
Sodium (Na+)	1–10	50–4000	5000–70,000	30,000–150,000
Calcium (Ca2+)	10–100	10–500	1000–50,000	5000–30,000
Magnesium (Mg2+)	1–5	10–200	50–3000	1000–10,000
Sulfate (SO42−)	1–10	50–2000	100–1000	100–500
Bicarbonate (HCO3−)	30–200	100–2000	100–500	10–300
Potassium (K+)	0.5–5	5–50	10–1500	500–5000

illustrates how mineral composition varies across different water types due to geological, chemical, and environmental factors. Freshwater is the least mineralized, while brine water represents the extreme end with the highest salinity. The plots shown in Figure 3 illustrate the complete data set of water chemistry regarding various types of water: fresh water, formation water, thermal–mineral water, and brines. The various water types in the Piper diagram (Figure 4a) are differentiated based on cation and anion compositions. Freshwater clusters around the Ca-Mg-HCO₃ region due to its low salinity values. On the other hand, formation water exhibits higher Na + K and Cl concentrations, indicating deeper geological interactions. Thermal–mineral waters have nearly equilibrated ionic composition, representing the geothermal component. Brines are very saline, Na-Cl-dominated waters (Figure 4b). Equilibration temperatures in the subsurface are also recorded: freshwater plots at lower temperatures, while thermal–mineral waters and brines plot along the higher geothermal temperatures (40 to 400 °C). The concentration plot displays ion concentrations: freshwater has the lowest ion concentrations, while brines exhibit highly concentrated ion levels (Figure 4c). The Langelier–Ludwig plot (Figure 4d) shows the relation of these ion groups: freshwater has low values, while brines are highly saline due to either evaporative or deep subsurface processes. Taken together, these plots give an idea of the chemical evolution and geochemical processes acting upon each water type.

High temperature and the unique chemical composition of thermal groundwater minerals firmly influence hydrocarbon behavior through changes in their solubility, mobility, and chemical stability. Increased temperature enhances hydrocarbon solubility and reduces viscosity, thus enhancing mobility and partitioning into aqueous and vapor phases. Furthermore, dissolved trace minerals and elements, such as sodium, calcium, bicarbonate, and chloride, influence hydrocarbon–water interactions by altering partition coefficients, which can lead to enhanced transport and redistribution across phases. High-temperature conditions also enhance chemical reactions, such as oxidation, hydrolysis, and thermal cracking, lowering hydrocarbons to smaller molecular weight and reactive forms. Moreover, dissolved trace metals in mineralized water can accelerate hydrocarbon polymerization or degradation through catalytic mechanisms.

Elevated temperatures greatly enhance the cross-solubility of hydrocarbons and water, impacting hydrocarbon transport and migration within geological environments. Enhanced solubility at high temperatures facilitates aqueous phase transport of hydrocarbons, a key parameter in hydrocarbon reservoir analysis as it allows for valid hydrocarbon tracing and assessment of migration pathways [115]. For example, in hydrothermal systems, elevated temperatures can redistribute fluid dynamics, which leads to improved hydrocarbon recovery [116].

The presence of various minerals combined with dissolved organic matter in groundwaters can cause changes to the biodegradation and transportation mechanisms of hydrocarbons. Petroleum-impacted areas display geochemistry involving dissolved organic components, whereby adsorption interactions between the latter and hydrocarbons might affect their degradation patterns [117]. Some hydrocarbons transform by reacting with natural organic matter in the groundwater, potentially inducing biogeochemical processes that either promote or inhibit degradation [27]. In addition, high levels of some minerals in groundwater have the potential to support structural alteration of hydrocarbons and consequently biochemical degradation processes. The outcome of these reactions largely depends on groundwater mineralogy [66].

In addition to the major ions presented in Table 2 and Figure 4, certain trace elements such as bromine, iodine, and boron are also relevant when interpreting geochemical processes in hydrocarbon-influenced groundwater systems. Trace elements such as bromide (Br-), iodide (I-), and boron (B) play key roles in determining the character of groundwater that interacts with hydrocarbon-bearing sequences. These elements will have high concentrations in formation waters and deep brines following prolonged water–rock interactions, dissolution of halites, and overburden pressure from organic-rich source rocks. Iodine and bromine are particularly excellent markers for the salinity source and evolution stage of sedimentary basins [85]. For example, high Br/Cl ratios may imply halite dissolution origin and low ratios may indicate evaporative concentration. High boron concentrations are frequently seen in formation water and are linked with the dissolution of organic matter or volcanic origins. These geochemical indicators can be used to monitor fluid mixing processes, identify hydrocarbon migration routes, and distinguish between natural and anthropogenic hydrocarbon sources in groundwater systems [118].

In the context of hydrocarbon contamination, the analysis of microelements like Br, I, and B can provide extra diagnostic tools other than standard parameters such as TDS, major ions, or BTEX compounds. Their destiny during hydrocarbon–water interaction will be governed by conditions including redox potential, salinity, and sorption. For instance, iodine has been proved to have a connection to the presence of thermogenic methane and thus could be employed as a tracer for natural gas seepage into aquifers. Boron, due to its mobility and content in oil-related waters, is widely used to assess the impact of oilfield activities on adjacent water bodies. These elements, when used in multivariate geochemical interpretation, enhance our ability to discern subtle traces of hydrocarbon influence in complex hydrogeological environments, especially where traces of direct organic pollution indicators can be lost or are absent [119].

6. Examples of Thermal–Mineral Groundwaters and Hydrocarbons

The presence of hydrocarbons within aquifers may exert a noteworthy influence on thermal–mineral water (Table 3), shown in a multitude of geochemical mechanisms and the progression of groundwater [120]. Specifically, hydrocarbons can cause changes in sediment ion exchange behavior, mineral dissolution and precipitation rates, and microbial activity in aquifers. Additionally, hydrocarbons influence the temperature and mineral content of groundwater by increasing the temperature and aiding recharge within the aquifer system [121]. This is important to know when assessing the response of the overall system to temperature changes and ensuring the sustainable use of groundwater for drinking and other purposes [7].

Hydrocarbons display chemical characteristics that render them suitable for interaction with thermal–mineral water as a result of their presence and behavior in hydrothermal settings. Various research studies (Table S2) have demonstrated that hydrocarbons, encompassing alkanes, aromatic hydrocarbons, and ketones, have been identified in the condensation products of excessively heated steam–water combinations and thermal water emanating from natural outlets [122]. Furthermore, utilization of the hydrocarbon–water–rock interaction experimental technique enables the precise replication of the evolution of hydrocarbon cracking and mineral modification within reservoirs, emphasizing the intricate organic–inorganic interaction processes. Moreover, examination of phase equilibria in immiscible binary systems such as water–hexane and water–pentane offers valuable insights into the compatibility of hydrocarbons with water across diverse temperature and pressure conditions [123]. These discoveries strongly indicate that hydrocarbons have the ability to coexist harmoniously and engage in interactions with thermal–mineral water, thereby demonstrating their remarkable chemical adaptability within hydrothermal environments [124]. The hydrocarbon extraction process, whether conventional or unconventional, has the potential to create risks and effects on water reservoirs, impacting both the quality of surface water and groundwater [6]. Geothermal waters that contain dissolved hydrocarbons, particularly methane, could allow uncontrolled methane emissions into the atmosphere, which can lead to explosion risks and potentially worsen the issue of global warming [125].

Table 3. Recent literature review of thermal–mineral springs and organic compounds.

A/A	Reference	Year	Location	Methodology	Type
1	Kárpáti Z. et al. [126]	1999	Hungary—Pannonia basin	Gas chromatography–mass spectrometry analysis of thermal water.	Thermal waters
2	Gioia M.L.D. et al. [127]	2006	Italy (Calabria)	Gas chromatography–mass spectrometry analysis of thermal water.	Thermal sulfurous waters
3	Gonzαlez-Barreiro C. et al. [128]	2009	Spain (Galicia)	Hydrochemical analysis, gas chromatographic analysis, and different extraction techniques (LLE, SPME, and SPE).	Thermal–mineral medicinal waters
4	Varga Csaba [129]	2012	Hungary	Gas chromatographic analysis and mass spectrometry.	Healing spa water
5	Kompanichenko et al. [122]	2016	Russia—Mutnovskii	Hydrochemistry and organic compounds.	Thermal waters
6	Kompanichenko et al. [130]	2022	Russia—Kuriles and Kamchatka Peninsula	Investigation of extractable organic compounds in hydrothermal vent fluids. Thermodynamic modeling of water–rock systems. Comparative analysis of organic matter in thermal waters.	Thermal waters

Table 3. Recent literature review of thermal–mineral springs and organic compounds.

A/A	Reference	Year	Location	Methodology	Type
1	Kárpáti Z. et al. [126]	1999	Hungary—Pannonia basin	Gas chromatography–mass spectrometry analysis of thermal water.	Thermal waters
2	Gioia M.L.D. et al. [127]	2006	Italy (Calabria)	Gas chromatography–mass spectrometry analysis of thermal water.	Thermal sulfurous waters
3	Gonzαlez-Barreiro C. et al. [128]	2009	Spain (Galicia)	Hydrochemical analysis, gas chromatographic analysis, and different extraction techniques (LLE, SPME, and SPE).	Thermal–mineral medicinal waters
4	Varga Csaba [129]	2012	Hungary	Gas chromatographic analysis and mass spectrometry.	Healing spa water
5	Kompanichenko et al. [122]	2016	Russia—Mutnovskii	Hydrochemistry and organic compounds.	Thermal waters
6	Kompanichenko et al. [130]	2022	Russia—Kuriles and Kamchatka Peninsula	Investigation of extractable organic compounds in hydrothermal vent fluids. Thermodynamic modeling of water–rock systems. Comparative analysis of organic matter in thermal waters.	Thermal waters

In Hungary’s Pannonian Basin, hydrocarbons such as alkanes and aromatic compounds have been detected in thermal spa waters. Kárpáti et al. [126] used GC–MS analysis to reveal the presence of low-molecular-weight hydrocarbons in waters used for therapeutic purposes, indicating a link between deep petroleum systems and shallow geothermal aquifers. Similarly, in Galicia, Spain, González-Barreiro et al. [128] analyzed thermal–mineral medicinal waters and found a variety of dissolved organic compounds, including petroleum-related hydrocarbons, suggesting upward migration through fault systems.

Kompanichenko et al. [122] reported the detection of hydrocarbons in natural thermal springs situated within a volcanic setting in Mutnovskii, Russia. Their findings emphasize the role of magmatic heat and fractured volcanic rocks in facilitating the transport of organic compounds from deep sedimentary basins to the surface. These interactions were confirmed through hydrochemical and organic compound profiling, pointing to both abiotic synthesis and migration from deep petroleum systems.

A case from Italy’s Calabria region involves thermal sulfurous waters that exhibited volatile hydrocarbon content, including methane and heavier fractions. Gioia et al. [128] emphasized that such compounds likely originated from deep thermogenic sources and were transported along major fault zones, highlighting the influence of regional tectonics and stratigraphy.

Tectonic activity is key to the formation and movement of thermal–mineral waters and directly shapes their chemical composition. Geological features like faults and fracture zones are the pathways for groundwater to penetrate deep into the Earth where it warms and becomes enriched with minerals from the surrounding rocks. This process allows the water to move upwards to the surface bringing with it trace elements like sodium, potassium, magnesium, and calcium, as well as other minerals like boron and lithium that make up the water’s unique chemical profile [105]. Tectonic forces and the geological features of the area influence not only the temperature but also the hydrochemical composition of thermal–mineral waters.

In Greece, this is more pronounced due to the country’s high tectonic activity especially along the Hellenic Arc where the African and Eurasian tectonic plates meet. As water descends it warms and becomes mineralized through contact with rocks containing calcium, magnesium, boron, and lithium [131]. When it resurfaces along fault lines it often emerges as thermal–mineral water with a unique chemical composition that has been used for therapeutic purposes for centuries. To trace the origin and recharge process of thermal–mineral waters in the Kyllini area in W. Greece, it is crucial to understand the geological, hydrochemical, and isotopic properties. By studying tectonic forces, water chemistry, and isotopic composition, research has shown that tectonic activity, especially fault systems, allows deep circulation and upwelling of mineral-rich, heated waters. Samples collected and analyzed over a year show that the thermal water of Kyllini with Na-Cl-HCO₃ composition remains stable with minor seasonal changes. Isotopic analysis also suggests a common origin of these waters with recharge that is observed in nearby semi-mountainous areas. Thermo-mineral waters can influence hydrocarbon generation and migration through various geological processes and provide valuable information for energy resources [132].

7. Future Challenges and Current Trends

The analysis of hydrodynamic, palaeo-hydrogeological and hydrochemical data can identify the hydrogeological situation of a region and reveal the conditions for the accumulation and preservation of hydrocarbon formations [133]. Thermo-mineral groundwater and hydrocarbons in oil fields are connected through various processes. In some cases, water and gas from oil-bearing formations mix with groundwater, as indicated by salinity tracers and isotopic compositions [38]. Additionally, in some oil fields, the mixing of fresh karstic groundwater with oil-field brines can occur, leading to increased water salinity and the addition of hydrocarbon and sulfur contaminants into the groundwater [134]. In the Anyue gas field in the Sichuan Basin, China, gas–groundwater interaction has been observed, suggesting the transport of hydrocarbons by groundwater [104]. However, in the Oxnard oil field in California, no evidence of water from hydrocarbon-bearing formations mixing with groundwater has been found [107]. In the Masjed Soleyman oil field in Iran, the mixing of fresh karstic groundwater with oil-field brines has been observed, leading to an increase in water salinity and the addition of hydrocarbon and sulfur contaminants [135].

In karst areas, hydrocarbons can migrate from high hydrocarbon potential strata to shallow groundwater through corrosion and extraction emissions [136]. Water–rock interaction plays a significant role in hydrocarbon emission into groundwater, with salinity acting as the main driving force [44]. The concentration of hydrocarbons in groundwater can be estimated by the mineral constituents, particularly concentrations of Ca²⁺ and Mg²⁺ [137]. In arid and semi-arid regions, rock-weathering/rock–water interaction is the dominant mechanism controlling groundwater, with the chemical relationships indicating a Ca²⁺-Mg²⁺-HCO₃⁻ water type [115]. In karst fissure groundwater systems, petroleum hydrocarbon pollution leads to hydrogeochemical mechanisms such as desulfurization and ion exchange. On the other hand, groundwater is also a key player in the exploration and recovery of hydrocarbons. It is often detected while drilling for oil and gas and deems oil and gas extraction difficult; however, it does provide much data on the local rock formations. The study of groundwater in hydrocarbon reservoirs is important because it is imperative for optimal resource management and highest possible recovery efficiency.

The contact between thermal mineral groundwater and source rock is vital in defining the behavior of hydrocarbons within sedimentary basins. These processes can be shown to be controlled by the generation, expulsion, and thermal maturation of hydrocarbons and are dependent on temperature, fluid composition, and geological characteristics of the source rock itself.

Heated mineral water can potentially assist in thermal maturation of source rocks and consequently facilitate hydrocarbon formation from organic matter. As illustrated in the case of a study on the Carboniferous shale reservoir of the Qaidam Basin, the tectonic subsidence and thermal condition evolve in synchronization with the processes of hydrocarbon generation in source rocks, illustrating groundwater’s capability to improve the maturation processes of kerogen transformation into hydrocarbons [138]. Additionally, thermal evolution of source rocks—triggered by geothermal gradients and availability of mineral-bearing fluids—establishes conditions that are conducive to hydrocarbon expulsion. Fluid expulsion due to interlayer water in source rocks enhances hydrocarbon expulsion during thermal maturation [139].

The contribution of hydrothermal fluids to hydrocarbon generation cannot be overstressed. It has been found from studies that high temperatures of such fluids can trigger the conversion of organic matter in source rocks and increase hydrocarbon generation rates markedly [140]. Hydrocarbons tend to move into and saturate fracture systems, where they may continue to interact with heat-altered minerals and solidify as low-reflectance solid bitumen. Hydrocarbon migration dynamics are also controlled by fluid flow and pressure alteration in mineral-bearing groundwater, causing intricate hydrocarbon accumulation and distribution behavior [139].

In addition, igneous activity can further affect the interactions between thermal groundwater and source rocks. The heat effect of magmatic intrusions will enhance hydrocarbon generation by hydrothermal pressurization and facilitate hydrocarbon expulsion from source rocks [141]. Injection of various minerals from magma into nearby sedimentary layers may trigger hydrocarbon generation and modify the geochemical conditions, which will influence the rate and effectiveness of hydrocarbon expulsion [142]. This contact between thermal mineral water, source rocks, and possibly intrusive igneous bodies is vital in determining the total hydrocarbon potential of an area. In general, thermal mineral groundwater interaction with source rock is a multifaceted process influencing hydrocarbon behavior directly. Temperature, the geochemical nature of groundwater, and the geological history of source rock all come together to influence hydrocarbon generation and migration.

A brief description of the drawbacks of the analytical methods for groundwater analysis is presented within this paper. However, a deeper analysis of the specific advantages and disadvantages of analytical methods to detect the hydrocarbons in groundwater is essential to select the optimal method for the specific condition of every site.

Apart from the environmental and geochemical applications demonstrated here, additional research directions can be focused on the integration of hydrocarbon–groundwater interactions into broader subsurface engineering operations, such as carbon geo-sequestration (CGS) and hydrogen storage. Thermal–mineral groundwater flow systems will more likely exist in deep geological formations that are otherwise being considered for long-term CO₂ or energy gas storage. Understanding the overall behavior of hydrocarbons, hot fluids, and injected gases under high temperature and pressure conditions is important to reservoir stability, geochemical stability assessment, and long-term environmental impact. Although this review primarily deals with natural and anthropogenic hydrocarbon processes in thermal–mineral waters, these findings can serve to guide future multidisciplinary research at the nexus of groundwater geochemistry and underground energy storage.

8. Conclusions

The dynamic relationship between groundwater and hydrocarbons underscores the necessity for comprehensive scientific studies and innovative technologies. Robust monitoring systems and preventive measures are essential to mitigate the risks associated with groundwater contamination by hydrocarbons. Furthermore, sustainable management practices are required to balance the utilization of hydrocarbon resources with the preservation of vital groundwater reservoirs. Studying the correlation between hydrocarbons and groundwater holds immense significance. The majority of publications focus on oil extraction sites and the monitoring of groundwater to prevent hydrocarbon leakage. The presence of hydrocarbons in groundwater can result in substantial environmental risks as contamination can have detrimental effects on human health through consumption.

This study provides an overview of natural hydrocarbon seepage in groundwater and thermal–mineral springs and the conclusions are summarized as follows:

• Formation water may help identify the process of hydrocarbon mixing with thermal–mineral waters, revealing a fingerprint of surface hydrocarbons.
• Analysis of groundwater quality and contamination is vital for its extraction, utilization, and safeguarding, particularly in regions with petroleum and petrochemical industries.
• Monitoring the salinity of groundwater can provide valuable insights into the potential presence of oil in a specific region.
• The interaction between water and hydrocarbons can affect the migration and accumulation of hydrocarbons in tight formations.

Hydrocarbon contamination in groundwater can have a profound impact on industries reliant on clean water sources. Groundwater is an indispensable resource for drinking, agriculture, and industry; thus, it is imperative to ensure its quality. Additionally, research into these interactions drives the advancement of innovative technologies and methodologies for the detection, monitoring, and remediation of hydrocarbon-contaminated groundwater, thereby contributing to the enhancement of environmental management practices. A summary of field-site literature is provided in Supplementary Table S1 [143,144,145,146,147,148,149,150,151].